1. Field of the Invention
The present invention relates to image forming technology.
2. Description of the Related Art
With a conventional image forming apparatus, such as a laser printer, a digital multifunction peripheral, or the like, that forms images using a laser beam scanning unit, images are formed using processes for increasing resolution, expressing multiple tones, smoothing, and so on, in order to achieve high image quality.
As representative methods for increasing the quality of an image, there are methods, called pulse width modulation (abbreviated hereinafter as “PWM”), luminance modulation, and so on that are performed on the image data, in which the darkness and the like of dots are controlled by controlling the strength (performing strength control) of the amount of light irradiated onto a photosensitive member, on a pixel-by-pixel basis. In recent years, PWM and luminance modulation are being combined in an attempt to further improve the tonal expression of the dots to be formed on a pixel-by-pixel basis (formed-pixel dots). Various developments are being made with respect to tone, image clarity, and the like, by altering the position, center, and so on of the formed-pixel dots. This results in improved, higher image quality.
(Pulse Width Modulation)
With regards to PWM techniques, there is a configuration that uses a reference clock of n times the frequency of an image clock used to define a single pixel (see Japanese Patent Laid-Open No. 8-321952). According to the controller illustrated in FIG. 1 of Japanese Patent Laid-Open No. 8-321952, a pixel pattern for expressing multiple tones is expanded in a look-up table in order to convert inputted multi-tone input image data into multi-tone image data for an image forming apparatus. The data is written into a video band buffer, after which it is set as binary data in serial order of the formed pixels using a parallel-to-serial converter; the resultant is then transferred as image data (VDO) for the image forming apparatus through an image clock (4VCLK) that is inputted sequentially.
At this time, the image clock (4VCLK) is configured as a multi-value clock having a value four times that of a reference clock (VCLK). In other words, 4-bit PWM having multi-valued (4-bit) tone, the 4 bits being realized by dividing a single pixel into 4 parts, is carried out, thereby expressing 16 tones. If, for example, the PWM data is 8-bit data, it is possible to express 256 tones. With PWM, a reference clock that is a multiple of the image clock is necessary, and the resolution and reference clock are in a proportional relationship. Furthermore, a horizontal synchronization signal for synchronizing the phase of each line image, and the synchronization accuracy, are both important factors.
In Japanese Patent Laid-Open No. 8-321952, there are no descriptions regarding the control of the centers of formed-pixel dots; however, it is possible to control the position of dots of equal width formed within a pixel by controlling the formation process for expressing multiple values within a single pixel to widen the pixel from the right side, from the left side, or from the center.
(Luminance Modulation)
Regarding luminance modulation techniques, the potential of a latent image formed on a photosensitive member can be controlled by controlling the strength of the light irradiated onto that photosensitive member. In recent years, it has become possible to carry out such luminance modulation along with PWM control on a formed-pixel dot basis, using high-frequency modulation techniques. Controlling the luminance on a pixel-by-pixel basis, or in other words, controlling the light source on a pixel-by-pixel basis by synchronizing the luminance modulation with PWM in order to further improve tonal expression, has been proposed (see Japanese Patent Laid-Open No. 2001-119648).
When the number of tones is increased in order to improve the tonal expression, the clock frequency also increases. When forming pixel dots of extremely small widths using PWM, there is a limit on the switching response of the light source, which is a laser or the like. As a response to a problem in which a predetermined pulse width for forming pixel dots of extremely small widths cannot be obtained, Japanese Patent Laid-Open No. 2001-119648 discloses a configuration that performs luminance modulation in synchronization with PWM processing. In other words, the technique of Japanese Patent Laid-Open No. 2001-119648 attempts to improve the expression of pixel dots formed on a photosensitive member by making it possible to set the strength of the luminance through luminance modulation at the time of PWM conversion executed on a formed-pixel dot basis. Through this, how smooth the change in the tonal strength is (the gradation) in the image formed on the photosensitive member can be expressed with increased smoothness.
Meanwhile, the capabilities of image forming apparatuses that use optical scanning have reached the level of 2400 dpi (dots per inch) in resolution and a print speed of 80 ppm (pages per minute). As the performance of image forming apparatuses increases, various proposals are being made to realize a further improvement in image quality, one of which is position correction for formed-pixel dots.
In order to form high-resolution pixel dots on a photosensitive member, it is necessary for pixel dot phasing to be aligned with high accuracy between the pixel dot array in the optical scanning direction (called “main scanning direction” hereinafter) and each optical scanning line (called “sub-scanning direction” hereinafter). Position correction control, which corrects the position of the formed-pixel dots in the main scanning direction and sub-scanning direction, respectively, has been proposed as a way to realize this.
A configuration in which the output clock of a PLL synthesizer that generates a reference clock for PWM is operated at a maximum oscillation frequency is disclosed in Japanese Patent Laid-Open No. 2004-249497. Japanese Patent Laid-Open No. 2004-249497 discloses continuously operating a PLL synthesizer that generates a clock having a higher frequency than an image clock and serving as a reference clock at a maximum oscillation frequency, and correcting the position, width, and so on of the pixel dots to be formed using a higher-frequency modulation unit.
Meanwhile, there is an image forming unit that exposes a subject to plural light sources, resulting from the emergence of multi-laser elements (VCSEL and so on). In order to form an image through scanning by controlling the flashes of plural light-emitting elements disposed in a matrix, it is necessary to temporally adjust individual element driving control and arrange the array of pixel dots formed on the photosensitive member. There has also been a proposal regarding pixel dot array control in which the correction of formed-pixel dot positions has been applied (see Japanese Patent Laid-Open No. 2006-175646).